1. Upgrade the factory.bin through the factory web interface or the u-boot
failsafe interface.
The firmware will boot up correctly for the first time.
Do not power off the device after OpenWrt has booted. Otherwise the u-boot
will enter failsafe mode as the checksum of the firmware has been changed.
2. Upgrade the sysupgrade.bin in OpenWrt.
After upgrading completes the u-boot won't complain about the firmware
checksum and it's OK to use now.
3. If you powered off the device before upgrading the sysupgrade.bin, just
upgrade the factory.bin through the u-boot failsafe interface and then goto
step 2.